Near Source Acoustical Particle Velocity Measurements with Ambient Noise

An acoustical measurement very near a structure can be a cheap alternative to other contactless vibration measurement techniques such as laser vibrometry. However, measurements of the acoustical pressure suffer greatly from ambient noise, making these measurements unsuitable for many industrial applications. De Bree and Druyvesteyn suggested that a measurement of the acoustical particle velocity does not have this drawback and provided theory and qualitative measurement results [1]. We present quantitative measurement results of the relative noise contribution in pressure and particle velocity measurements. The ratio of these quantities receives special attention. The model used in previous research is a lumped model, in which some important aspects are neglected. We present results of a numerical model of the vibrating structure and the air. The numerical model and the measurements indicate the same trends but the lumped model does not describe the trends well. Nevertheless, this study also suggests that the sensitivity to background noise is generally considerably greater in pressure measurement than in a particle velocity measurement.